Protective equipment for a series capacitor

ABSTRACT

Protective means for a series capacitor comprises a normallyopen triggered vacuum interrupter that is triggered into conduction to establish a bypass circuit shunting the capacitor in response to a condition producing an overvoltage across the capacitor. After the interrupter is so triggered, its contacts are driven into engagement to provide a solid metallic path for continued current flow through the bypass circuit. When the bypass circuit current subsides to a predetermined level, a resistor is inserted into the bypass circuit in series with the closed contacts of the interrupter, shortly after which said contacts are separated to interrupt the bypass circuit. Any restrike occurring in the vacuum interrupter during the latter interruption produces a low-current discharge through the series combination of the resistor and the interrupter that acts to clean up the contacts of the interrupter.

United States Patent 191 Johnson et al.

Aug. 7, 1973 PROTECTIVE EQUIPMENT FOR A SERIES CAPACITOR Inventors: Ingolf B. Johnson, Schenectady,

N.Y.; Graham R. Mitchell, Willingboro, N.J.

General Electric Company, Philadelphia, Pa.

Filed: Sept. 5, 1972 Appl. No.: 286,469

Assignee:

References Cited UNITED STATES PATENTS CONTACT OPERATOR Primary Examiner-J. D. Miller Assistant Examiner-Harvey Fendelman Attorney-J. Wesley Haubner and William Freedman [57] ABSTRACT Protective means for a series capacitor comprises a normally-open triggered vacuum interrupter that is triggered into conduction to establish a bypass circuit shunting the capacitor in response to a condition producing an overvoltage across the capacitor. After the interrupter is so triggered, its contacts are driven into engagement to provide a solid metallic path for continued current flow through the bypass circuit. When the bypass circuit current subsides to a predetermined level, a resistor is inserted into the bypass circuit in series with the closed contacts of the interrupter, shortly after which said contacts are separated to interrupt the bypass circuit. Any restrike occurring in the vacuum interrupter during the latter interruption produces a low-current discharge through the series combination of the resistor and the interrupter that acts to clean up the contacts of the interrupter.

10 Claims, 2 Drawing Figures CONTACT OPERATOR PAIENIEM 3.751.716-

l l I J4 6'0 CONTACT con/mar J0 OPERA TOR OPERA TOR denly increase due to a fault on the power system, the

\ line immediately after this fault removal, as this is the 1 PROTECTIVE EQUIPMENT FORA SERIES I CAPACITOR A G OUN o a. This inventionr'elates to means for protecting a ca- 5 pacitor that is connected in series with a power line from overvoltages resulting from excessive line current therethrough. Examples of this type of capacitor-protective equipment are disclosed in the-following U.S. Pat. Nos:

nect a normally non-conductive gap device in parallel with the series capacitor. If the line current should sudvoltage across the series capacitor will rise abruptly toward an excessive value-The gap device, however, is designed to break down before this voltage reaches a damaging value and, in so doing, to establish a low impedance shunt circuit around the series capacitor through which the excessive current can flow without developing excessive voltage across the series capacitOt.

When the fault responsible for the excessive current is isolated, as by the opening of a circuit breaker located between the fault and the power line, the line current returnseither to its normal value or to a higherthan-normal emergencyoperating value. It is important that the series capacitor be reinserted intothe power timeits presence is most needed for the purpose of maintaining stability and voltage control of the power system.

Prior schemes for removing and later reinserting the capacitor in this manner have been subject to a number of disadvantages. Typically, they have been quite complicated and expensive and, even with their complications, have not been as precise and quick in their operation as mightbe desired.

A relatively simple and inexpensive way of performing the required functions is described and claimed in the aforementioned U.S. Pat. No. 3,3I9,I2l-'Lee, assigned to the assignee of the present invention. There the protective device is a triggered vacuum circuit interrupter connected in the shunt or bypass circuit. When the voltage across the capacitor approaches an excessive value, thetrigger of this device is operated to produce an arc-over between the then-open'contacts of the device, thereby completing the bypass circuit. Immediately following such arc-over, the contacts of the triggered vacuum interrupter are driven into engage ment to extinguish the high current are therebetween and provide a solid metallic path for carrying the current then flowing through the bypass circuit. When the current through this bypass circuit (i.e., line current) later subsides, as when the fault is isolated, the contacts of the triggered vacuum interrupter are separated to of the interrupter, shortly after whichsaid contacts are interrupt the current then flowing through the bypass circuit.

One problem encountered with such an arrangement is that the vacuum interrupter sometimes has difficulty in interrupting the bypass circuit when its contacts are separated. Although the current then being interrupter is usually relatively light, a restrike sometimes occurs between the contacts during interruption or shortly thereafter; and this restrike can lead to an erroneous removal of the capacitor from the power line.

The probable cause of such a restrike is that the contacts of the vacuum interrupter arethen in a relatively poor condition as a result of the heavy current arcing that had occurred between them prior to their being driven into engagement following initial completion of the bypass circuit. This poor condition of the contacts can render the triggered vacuum interrupter incapable of continuously withstanding the voltage present between its then-separated contacts.

SUMMARY An object of the present invention is to construct the protective arrangement in such a manner that even if such restrikes do occur, they will not result in erroneous removal of the capacitor from the power lines.

Another object is to render the triggered vacuum interrupter capable of interrupting the bypass circuit and of maintaining the circuit open despite the occurrence of one or more of such restrikes.

Still another object is to provide a protective arrange-- ment which, though it has the'above insensitivity to relow impedance bypass circuit should the voltage across the capacitor at any time approach an excessive value.

' In carrying out the invention in one form, we provide a normally-open triggered vacuum interrupter in the bypass circuit shunting the capacitor. After this interrupter has been triggered into conduction in response to a condition producing an overvoltage across the capacitor, thereby completing the bypass circuit, its contacts are driven into engagement to provide a solid metallic path for continued current flow through the bypass circuit. When the bypass circuit current has sub sided to a predetermined level, a resistor is inserted into the bypass circuit in series with the closed contacts separated to interrupt the bypass circuit. Any restrike occurring in the vacuum interrupter during the latter interruption produces a low-current discharge through the series combination of the resistor and the interrupter that acts to clean up the contacts of the interrupter.

BRIEF. DESCRIPTION OF DRAWING 1 DETAILED DESCRIPTION OF PREFERRED EMBODIMENT Referring now to the drawing, there is shown a high voltage alternating current transmission line 10 and a the protective series capacitor 12 connected in series with line 10. For protecting series capacitor 12 from overvoltages that could result fromexcess current therethrongh, a protective arrangement 13 is connected in parallel with capacitor 12.

As shown in the schematic diagram of FIG. 1, this protective arrangement 13 comprises two triggered vacuum circuit interrupters 14' and 114 connected in series with each other in a bypass circuit 15 that is connected in parallel'with series capacitor 12. One of the triggered vacuum circuitinterrupters'14 is a normallyopen device, referred to hereinafter as the main interrupter; and the other 114' is a normally-closed device, referred to hereinafter as the auxiliary interrupter. Connected in parallel with the normally-closed auxiliary interrupter 114 and in series with the normallyopen main interrupter 14 is a resistor 17 which is effectively shorted out by. the normally-closed auxiliary interrupter 114. The manner in which these components operate and cooperate will soon be described in detail.

, Referringto FIG. 2, the rnaininterrupter 14 is preferably a triggered vacuum interrupter of the general type shown and claimed in the, aforesaid U.S. Pat. No. 3,3 19,12 1 --Lee. Accordingly, it comprises a sealed en velope -16that is evacuated to a pressure of 10"jmm. of mercury or lower. The envelope 16 comprises a casing 18 of a suitable insulating material, suchas a ceramic, and a pair of metallic end caps 20 and 21 joined in vacuurn tight relation to the respective oppositeends of theinsuIat'ingcas'ing-IS by suitable seals 22.

Located within the evacuated envelope 16 is a pair tionship along the length of the support. These two layers of metal constitute the electrodes of the trigger gap. They are separated by a V-s'haped groove 34 that extends about the circumference of the ceramic support" and has its walls defined by the ceramic material itself.

One of the trigger electrodes 36 is electricallyconnected to the main electrode 24. The other electrode 30 is normally electrically isolated from the main elecsageway in the' ceramic support 32. At its inner end,

this lead 38 is brazed to a metallic cap 37 which is in electrical contact with the trigger electrode 30. The

' metallic cap 37 is hermetically sealed to the inner end of the ceramic support 32 by conventional metal-toce'ramic sealing techniques so as tomaintain thehermetic seal'of the envelope. r I 7 .For applying a triggering pulse to the trigger gap 29, a-pulse source 40, with output leads ,42 and 43,- is provided. When pulse source 40 operates, it applies a voltage pulse between the trigger electrode 30 and 36 that establishes a small arc across trigger gap.29. This are vaporizes some of the material of electrodes 30 and 36.

' These vaporsare rapidly propagated, or ejected, into of main electrodes 24 and 25 that are normally spaced apart to define a main or primary gap 26 located therebetween. These electrodes are preferably of a diskshape configuration. Electrode 24 is a stationary electrode which is supported on the upper end plate 20 by means of -a tubularsupporting rod 24a; whereas electrode 25 is a movable electrode which is joined to and carried by an elongated conductive operating rod 25a that projects through an opening in the lower end-plate 21. A flexible metallic bellows 27 is provided about the operatingrod 25a to permit vertical movement thereof without impairing the vacuum inside the: evacuated envelope 16. Thisbellows 27 is secured by suitable seals at its respective opposite ends to the operating rod 25a and the end'plate 21. v

For controllingithe position of the movable contact 25 of 'main interrupter 14, suitable operating means 50,

shown'for simplicity in' block form, is provided. This operating means 50 comprises means (not shown) for normally maintaining the-movable contact 25 in its open position ofF lG. 2, immediately upon receiving a closing signal via a channel schematically shown at 54, operator 50 responds to quicklydrive contact 25 in an upward closing direction into engagement with contact For causing the primary gap 26 to arc-over under certain conditions soon to. be descfibed, there is pro- 'vided atrigger gap; 29 located within a centrallydisposed recess31 provided in stationary electrode 24. This trigger gapis preferably constructed in generally the manner. disclos'edand claimed in US. Pat No.

3,08 7',092-Lafferty,' assigned to theassignee of the present invention. Accordingly, it comprises a cylindrical ceramic support 32 located within recess 31 and two thin layers 30 and 36 of metal bonded to theexternal surface of the ceramic support in spaced-apart relathe primary gap 26, thus drastically reducing its dielectric strength and causing it to arc-over in response to the voltage then prevailing between main electrodes 24 and'-25. The above-described metallic vapors are referred to hereinafter as charged conduction carriers.

Main electrodes 24 and 25 are made of a non.-

. refractory metal, such as copper,-that-is substantially evolves no appreciable quantity of non-condensable gap 26. This condensation of these'metallic vapors oc-, curs very rapidly, and thispermits'the gapto build up.

free of all gaseous. impurities and impurities which, upon decomposition, will produce gases. Accordingly," thearcthatis established between the main electrodes gases from the main electrodes. This aids the main gap in recovering its dielectric strength immediately after'a current zero is reached. t

The are across the main gap does vaporize metal from the mainelectrodes, but these are metallicvapors that can be readily condensed. For this purpose, a tubular metallic shield 49 is provided about the main gap 26 to intercept and condense the arc-generated metallic vapors as they are projected radiallyoutward frornthe itsdielectric strength at a very high rent zero point is reached.

When current zerois reached at the end of a half cycle of arcing current, dielectric strength can be built up across the main gap 26at such a high rate that the breakdown voltage on the next half cycle of current is rate when the curagain determined by the trigger gap rather than the main gap. lf, following current zero, the pulse source supplies another pulse to the trigger gap 29, the trigger gap again sparks over, thus initiating another arc-over of the primary gap 26 andfcausing currenttocontinue flowing until at least the next natural current zero. v

For preventing the gap device from being damaged or otherwise impaired by unduly prolongedarcing, we

cause the operating means 50 to rapidly drive the movable contact into engagement with stationary contact 24 immediately following an arc-over of primary gap 26 by the trigger means. Such contact engagement establishes a solid metallic path between the contacts 24, 25, thereby extinguishing the are previously presem. 1

For causing operating means 50 to perform in this manner, a signal channel 54 is provided between operating means 50 and pulse source 40. When pulse source 40 supplies a pulse to trigger gap 29, a signal is transmitted over this channel 54 to operating means 50. Operating means 50 responds to the signal to drive movable contact 25-into engagement with stationary contact 24, as above-described.

In a preferred embodiment, the auxiliary circuit interrupter 114 is also of the triggered vacuum interrupter type. Since auxiliary circuit interrupter 114 is substantially identical to main interrupter 14, no detailed description thereof is considered necessary. Cor

responding parts of the two interrupters have been assigned similar reference numerals, differing only in that the prefix 1 appears ahead of those numerals used for auxiliary interrupter 114.

Auxiliary interrupter 114 is a normally-closed device having its movable contact 125 normally maintained in engagement with stationary contact 124. The position of movable contact 125 is controlled by suitable contact operating means 60, schematically shown in block form. When operating means 60 receives an opening signal via a channel schematically shown at 62, it responds by immediately driving contact rod 125a downwardly to separate movable contact l25 from stationary contact 124.

lf current is then flowing through interrupter 114,

separation of its contacts draws an arc therebetween.

This are persists until a natural current zero, at which time the arc is prevented from reigniting by the high dielectric strength of the vacuum.

To aid in condensing the metal vapors generated by such arcing so as to permit a rapid recovery of dielectric strength, a shield 149 is provided. This shield serves the same purpose as shield 49 of interrupter l4.

lnterrupter 114 includes a' trigger gap 129 corresponding to trigger gap 29 of interrupter 14. For sparking over this trigger gap 129 in response to predetermined conditions, a pulse source 140 is provided having output leads 142 and 143 connectedacross trigger gap 129 via parts138 and 120.

- The two pulse sources 40 and 140 are arranged to op- I crate in response to the voltage across capacitor 12 reaching a predetermined level. In this connection, three high impedance voltage-dividing capacitors 150, I51, and 152 are shown connected across series capacitor 12 so that the voltage appearing across each is proportional to the voltage acrosscapacitor 12. The voltage across one capacitor 150 is fed to pulse source 140 as input information and that across 152 is fed to pulse source 40 as input information. When these voltages exceed a suitable value, predetermined for each pulse source, the pulse source operates to develop an output pulse.

Assume now that the protective arrangement 13 is in its condition of FIGS. 1 and 2 and that a fault develops on line 10. This will cause a rapid increase in line current, which abruptly increases the voltage across capacitor 12. When this voltage reaches a predetermined level, pulse source 40 (FIG. 2) develops a voltage pulse which immediately sparks over trigger gap 29, which, in turn, immediately produces an arc-over of primary gap 26. This arc-over completes the bypass circuit 15 through the two interrupters 14 and 114, thus establishing a low impedance shunt around capacitor 12, which prevents a further increase in the voltage thereacross. Completion of this low impedance shunt 15 allows capacitor 12 to discharge therethrough thus producing a very high current in circuit 15, limited primarily by a reactor providing for current-limiting purposes.

As soon as primary gap 26 arcs-over as above described, contact-operating means 50 receives a closing signal via channel 54 and responds by driving contact 25 into engagement with contact 24. This extinguishes the are between the contacts and establishes a solid metallic circuit through interrupter 14. So long as the fault which initiated arc-over of interrupter 14 remains un isolated, the current through power line 10 and interrupter 14 is high. But when this fault is finally isolated, the current falls to a much lower level.

To sense this fall in current, a suitable current sensor is coupled to the bypass circuit 15 through a current transformer secondary winding 82. When the current falls to a predetermined level, current sensor 80 supplies an opening signal to contact-operator 60 for interrupter 114. Operator 60 responds by quickly operating to drive contact rod a downwardly, thereby separating the contacts .of interrupter 114, thus interrupting the circuit therethrough, as above described. This removes the short circuit from around the resistor 17, effectively inserting the resistor into the bypass circuit 15 in series with the then-closed contacts of interrupter 14. The resistor 17 is typically of such a size that only about 20 percent of the line current flows there'- through under these conditions. Accordingly, the capacitor 12 is effectively reinserted into the power circuit upon opening of interrupter 114 and is thus then available to help maintain stability.

Depending on contingencies involved and criteria applied, approximately 1% to 15 cycles after the interrupter 114 has been opened to insert the resistor 17 in series with the then-closed interrupter 14, interrupter 14 is opened to interrupt the bypass circuit 15. Such opening is effected by operator 50 acting on information supplied via a channel 83. When this interrupting operation is completed, the capacitor 12 is fully returned to the line 10 and all of the line current flows therethrough. Not infrequently, during this interrupting operation, restrikes, or reignitions, occur across the gap 26 of the interrupter 14. The probable cause of such restrikes is that the contacts of the interrupter 14 are then in a relatively poor condition as a result of heavy current arcing that had occurred between them prior to their being driven into engagement at the start of the capacitor-removal operation. Such heavy current arcing, especially when followed as it is by closing impact and resultant welding and by subsequent breaking of the weld on opening, often leaves the contacts,

with sharp protuberances. This poor condition of the contacts impairs the dielectric properties of the gap 26 and thus can render the vacuum interrupter incapable of withstanding the voltage present between its contacts.

Our protective arrangement not only can tolerate such restrikes but is actually improved bythem. In this respect, a restrike across gap 26 is not followed by flow of any great amount of current, as would be the case in certain prior art arrangements, but only by the current that can flow through resistor 17, which is then in series with interrupter 14 and no longer shorted out by auxiliary interrupter 114. This is a relatively low current which the interrupter can easily interrupt at the next current zero following the restrike.

The short-duration low current are produced by such a restrike is actually beneficial in that it cleans up the contacts of the interrupter 14 by burning off, or vaporizing, the above-described protuberances thereon, thus improving the dielectric properties of gap 26. Should one or more additional restrikes occur as a result of any remaining protuberances, these protuberances would be similarly burned off, or vaporized, by the resulting arc.

About 20 cycles after the interrupter 14 has been opened and the probability of a restrike in interrupter 14 has become negligible, the then-open interrupter 114 from the effects of prolonged arcing.

if such a restrike occurs whilethe auxiliary interrupter 114 isclosed, the low impedance bypass circuitIS (with resistor 17 shorted out) is reestablished around the capacitor l2, discharging the capacitor and erroneously removing it from the line. But if a restrike occurs while interrupter 114 is still open, the resistor 17 is available to limit the extent of capacitor discharge and to enable the main interrupter 14 to recover its dielectric strength on the next current zero following the restrike. I 1

1f resistor l7-was shorted out at the time-a restrike occurred, a veryhigh current, i.e., the full capacitor discharge current, would follow the restrike, and the main interrupter 14 would havegreat difficulty interrupting such high current at the next current zero. Even if interruption at this instant was successful, the contacts of .the main interrupter 14 would have been subjected to unnecessary-high current arcing and resultant impairment thereby. v

Another significantadvantage that results from inserting the resistor 17 in the bypass circuit prior to interruption thereof by main interrupter 14 is that the reinsertion transient voltages and possible trapped chargesjamong one or morecapacitor segments in a bank or one ormore capacitor banks are muchless severe than they would be if there was no preinsertion of resistance prior to opening of the interrupter 14. The ohmicvalue of the resistor determines the severity of the *reihsertion transient voltagesand of the trapped charges. Preferably, our resistor 17 has an ohmic value in the range of about three to five times the capative reactance (X )of thereinserted capacitor.

There is a-possibility that during the above-described period when the auxiliary interrupter is open, a fault or other over-voltage-producing condition could develop on power line 10. To protect the capacitor 12 from such overvoltage, it is important that the protective arrangement be capable of immediately reestablishing the low impedance bypass circuit around capacitor 12. Our protective arrangement has this capability because.

auxiliary interrupter 114 has triggering means (129,

140) which in response to such an incipient overvoltage can immediately trigger the then-open interrupter 114 into conduction, thus immediately reestablishing the low impedance bypass circuit. It is noted that this low impedance bypass circuit is established without waiting for the contacts 124, of the auxiliary interrupter 114 to be closed. If the main interrupter 14 is open at the time of this incipient overvoltage, its triggering means 29, 40 will also immediately trigger it into conduction so that the bypass circuit 15 will be completed when auxiliary interrupter 114 becomes conductive.

immediately following such triggering of auxiliary interrupter 114, the operator 60 thereof operates to engage contacts 124, 125 of interrupter 114 in response to asignal supplied via a channel 154. This quick closing operation by the operator 60 protects interrupter Each time one of the pulse sources or 40 supplies atriggering pulse to its associated interrupter, the pulse source also supplies a signal tothe operator of the interrupter, if the interrupter is open, commanding the contacts, it is to be understood that each interrupting device could comprise a plurality of series-connected pairs of separable contacts arranged for simultaneous operation in a suitable conventional manner. Such plural'pairs of series-connected contacts are needed when the circuit voltage is higher than that which can be handled by a single pair. One way of providing plural pairs of contacts is to construct each interrupting 'device of a plurality of separate'vacuum interrupter units, each as shown in the drawing, with means for mechanically interconnecting the movable contactrods of the interrupter units.

Although our perferred'. embodiment utilizes a trig gered vacuum interrupter 114 for the auxiliary inter-, rupter, other suitable types of interrupting devices can instead be utilized for this auxiliary interrupter duty. An example of another suitable type is the gap device disclosed and claimed in the above-mentioned U.S. Pat. No. 2,760,12l-Roth, assigned to the assignee er the present invention. When used for thisduty, the

- determined level, the gapdevice returns to it s'norrnally non-conductive state, inserting the resistorin series with themain interrupter 14, shortly after which the maininterrupter 14 opens to interrupt the bypass circuit current in the same manner as described hereinabove.

In another modified form of the invention, we use a non-linear resistor in place of the linear resistor 17 shown. Thezuseof such a non-linear resistor reduces the transient voltages developed when auxiliary interrupter 114 is opened to insert the resistor. Such nonlinear resisto'r'is of a material having a negative resistance-current characteristic; such as the material sold by General Electric Company under the trademark Thyrite. The non-linear resistor is sized to have a resistance equal to a value in the range ofthree to five times X atan applied instantaneous voltage equal to the voltage across the capacitor 12 that causes pulse source 40 to initiate arc-over of the main interrupterl4. X is the capacitive reactance of capacitor 12.

While we have shown and described a particular embodiment of our invention, it will be obvious to those skilled in the art that various changes and modifications may bemade without departing from our invention in its broader aspects; and we, therefore, intend herein to cover all such changes and modifications as fall within the true spirit and scope of our invention.

What I claim as new and desire to secure by Letters Patent of the United States is: I

1. Means for protecting a capacitor that is connected in series with a power line from overvoltages resulting from excessive line current therethrough, comprising:

a. a first interrupter of the triggered vacuum type comprising: an evacuated envelope, relatively movable contacts therein that are separable to establish a primary gap therebetween, and trigger means operableto produce an arc-over of said primary gap,

b. a second interrupter .having separable contacts connected in series with the contacts of said triggered vacuum interrupter in a bypass circuit for shunting said capacitor,

c. a resistor connected in parallel with the contacts of said second interrupter and in series with the contacts of said triggered vacuum interrupter,

d. means for normally maintaining the contacts of said second interrupter in a closed position for effectively shorting out said resistor, g e. means for normally maintaining the contacts of said triggeredvacuum interrupter in an open position for effectively blocking current through said bypass circuit, f. means responsive to a condition producing an overvoltageacross said capacitor for operating the trig T .ger means of said triggered vacuum interrupter to produce an arc-over-of its primary gap, thereby completing said bypass circuit through said first and second interruptersj g. means for engaging the contacts of said first interrupter immediately followingoperation of said trigger means to arc-over said primary gap, thereby providing a solid metallic path for current through said bypass circuit, h. means for separating the contacts of said second interrupter when the current through said power line subsides to a predetermined level, thereby inserting said resistor into said bypass circuit in series with the then-engaged contacts of said first interrupter,

. and means for separating the contacts of said first a. said second interrupter is also a triggered vacuum interrupter comprising: an evacuated envelope in which the separable contacts of said second interrupter are located, and trigger means operable to produce an arc-overbetween said separable contacts while said second contacts are separated, and

b. means is provided for operating the trigger means .said first interrupter have been maintained separated for a predetermined period, this restoring the protective means to its normal condition. 4. The interrupter of claim 1 in which any restrike occurring in said first interrupter following separation of its contacts while said second interrupter is open produces a low current discharge through the series combination of said resistor and said first interrupter that cleans upthe contacts of said first interrupter.

5. The interrupter of claim 4 in combination with means for restoring the contacts of said second interrupter to an engaged position after the contacts of said first interrupter have been maintained separated fora predetermined period sufficiently long to reduce to insignificance the probability that further restrikes will occur in the first interrupter until after the first interrupter is again caused to arc-over by operation of its trigger means.

6. The interrupter of claim 1 in which said resistor has an ohmic value in the range of three to five times the capacitive reactance of said capacitor.

7. The interrupter of claim 1 in which said resistor is a non-linear resistor having anohmic value in the range of three to five times the capacitive reactance of said capacitor at an instantaneous voltage applied to said resistor equal to the voltage across the capacitor that causes said trigger means to operate to produce an areover of said primary gap.

8. Means for protecting a capacitor that is connected in series with a power line from overvoltages resulting interrupter to interrupt said bypass circuit shortly after insertion of said resistor into said bypass circuit. 2. The protective means of claim 1 in which:

from excessive line current therethrough, comprising: a..'a first interrupter of the triggered vacuum type I comprising: an evacuated envelope, relatively movable contacts therein that are separable to establishv a primary gap therebetweemand trigger means operable to produce an arc-over of said primary gap, b. means for connecting said contacts in a bypass circuit for shunting said capacitor, c. means for normally maintaining the contacts of said triggered vacuum interrupter in an open posi-- tion for effectively blocking current through said bypass circuit,

d. means responsive to a condition producing an overvoltage across said capacitor for operating the trigger means of said triggered vacuum interrupter to produce an arc-over of its primary gap, thereby completing said bypass circuit through said first interrupter,

e. means for engaging the contacts of said first inter rupter immediately following operation of said trigger means to arc-over said primary gap, thereby a providing a solid metallic path for current through said bypass circuit, I f. a resistor associated with bypass circuit,

- g. means including a second interrupter shorting out interrupter to interrupt said bypass circuitshortly after insertion of said resistor into said cuit. r 1 9. The interrupter of claim 8 in which any restrike occurring in said first interrupter following separation of v its contacts while said secondinterrupter is open produces a low current discharge through the series comv bination of said resistor and said first interrupter that cleans up the contacts of said first interrupter.

10. The interrupter of claim 8 in which said resistor has an ohmic value in the range of three to five times the capacitive reactance of said capacitor.

bypass cir- 

1. Means for protecting a capacitor that is connected in series with a power line from overvoltages resulting from excessive line current therethrough, comprising: a. a first interrupter of the triggered vacuum type comprising: an evacuated envelope, relatively movable contacts therein that are separable to establish a primary gap therebetween, and trigger means operable to produce an arc-over of said primary gap, b. a second interrupter having separable contacts connected in series with the contacts of said triggered vacuum interrupter in a bypass circuit for shunting said capacitor, c. a resistor connected in parallel with the contacts of said second interrupter and in series with the contacts of said triggered vacuum interrupter, d. means for normally maintaining the contacts of said second interrupter in a closed position for effectively shorting out said resistor, e. means for normally maintaining the contacts of said triggered vacuum interrupter in an open position for effectively blocking current through said bypass circuit, f. means responsive to a condition producing an overvoltage across said capacitor for operating the trigger means of said triggered vacuum interrupter to produce an arc-over of its primary gap, thereby completing said bypass circuit through said first and second interrupters, g. means for engaging the contacts of said first interrupter immediately following operation of said trigger means to arcover said primary gap, thereby providing a solid metallic path for current through said bypass circuit, h. means for separating the contacts of said second interrupter when the current through said power line subsides to a predetermined level, thereby inserting said resistor into said bypass circuit in series with the then-engaged contacts of said first interrupter, i. and means for separating the contacts of said first interrupter to interrupt said bypass circuit shortly after insertion of said resistor into said bypass circuit.
 2. The protective means of claim 1 in which: a. said second interrupter is also a triggered vacuum interrupter comprising: an evacuated envelope in which the separable contacts of said second interrupter are located, and trigger means operable to produce an arc-over between said separable contacts while said second contacts are separated, and b. means is provided for operating the trigger means of said second interrupter to produce an arc-over between the separated contacts thereof should the voltage across said capacitor reach a predetermined level when said contacts of said second interrupter are separated.
 3. The protective means of claim 1 in combination with means for restoring the contacts of said second interrupter to an engaged position after the contacts of said first interrupter have been maintained separated for a predetermined period, thus restoring the protective means to its normal condition.
 4. The interrupter of claim 1 in which any restrike occurring in said first interrupter following separation of its contacts while said second interrupter is open produces a low current discharge through the series combination of said resistor and said first interrupter that cleans up the contacts of said first interrupter.
 5. The interrupter of claim 4 in combination with means for restoring the contacts of said second interrupter to an engaged position after the contacts of said first interrupter have been maintained separated for a predetermined period sufficiently long to reduce to insignificance the probability that further restrikes will occur in the first interrupter until after the first interrupter is again caused to arc-over by operation of its trigger means.
 6. The interrupter of claim 1 in which said resistor has an ohmic value in the range of three to five times the capacitive reactance of said capacitor.
 7. The interrupter of claim 1 in which said resistor is a non-linear resistor having an ohmic value in the range of three to five times the capacitive reactance of said capacitor at an instantaneous voltage applied to said resistor equal to the voltage across the capacitor that causes said trigger means to operate to produce an arc-over of said primary gap.
 8. Means for protecting a capacitor that is connected in series with a power line from overvoltages resulting from excessive line current therethrough, comprising: a. a first interrupter of the triggered vacuum type comprising: an evacuated envelope, relatively movable contacts therein that are separable to establish a primary gap therebetween, and trigger means operable to produce an arc-over of said primary gap, b. means for connecting said contacts in a bypass circuit for shunting said capacitor, c. means for normally maintaining the contacts of said triggered vacuum interrupter in an open position for effectively blocking current through said bypass circuit, d. means responsive to a condition producing an overvoltage across said capacitor for operating the trigger means of said triggered vacuum interrupter to produce an arc-over of its primary gap, thereby completing said bypass circuit through said first interrupter, e. means for engaging the contacts of said first interrupter immediately following operation of said trigger means to arc-over said primary gap, thereby providing a solid metallic path for current through said bypass circuit, f. a resistor associated with bypass circuit, g. means including a second interrupter shorting out said resistor during most of the period when current is flowing through said bypass circuit but operable when the current through said power line subsides to a predetermined level to insert said resistor into said bypass circuit in series with the still-engaged contacts of said first interrupter, h. and means for separating the contacts of said first interrupter to interrupT said bypass circuit shortly after insertion of said resistor into said bypass circuit.
 9. The interrupter of claim 8 in which any restrike occurring in said first interrupter following separation of its contacts while said second interrupter is open produces a low current discharge through the series combination of said resistor and said first interrupter that cleans up the contacts of said first interrupter.
 10. The interrupter of claim 8 in which said resistor has an ohmic value in the range of three to five times the capacitive reactance of said capacitor. 